This invention relates to a method for the production of primary, secondary or tertiary phosphines and to phosphines obtained from the aforesaid method.
In particular, the present invention relates to the preparation of organophosphines of the formula R(a)PH(3xe2x88x92a) where R is an organic group and a is 1, 2 or 3.
Organophosphines are used as intermediates in the synthesis of a wide range of organophosphorus fine chemicals, including pesticides and pharmaceuticals. One conventional method of preparing alkylphosphines involves the use of high-pressure phosphine. This is a hazardous operation and requires a special plant which is expensive to install, expensive to maintain and expensive to operate safely. Moreover, this technology is used to manufacture tri-alkyl phosphines. Primary and secondary phosphines are not readily accessible by this method.
A more selective alkylation of phosphine can be achieved by reacting Grignard reagents with phosphorus halides, followed by reduction with, for example, lithium aluminium hydride. However, this approach is generally unsuitable for commercial production on account of the high cost of the reagents.
Primary and secondary phosphines can also be prepared by thermal disproportionation of alkyl- or dialkyl-phosphonous acids. Such routes have low yields.
We have now discovered that primary, secondary or tertiary phosphines can be prepared by way of a convenient reaction using relatively low cost reagents and standard pressures. In particular we found that the reaction can be made to yield specific mono-, di-, or tri-alkyl phosphines in high yield, or desired mixtures of the phosphines.
Accordingly, the present invention provides a method for the production of a primary, secondary or tertiary phosphine having the general formula (R)aP(H)3xe2x88x92a, where a is 1, 2 or 3, in which the method comprises the following stages:
(i) reacting a tris(hydroxyorgano)phosphine (THP) with an organic halogen-containing compound;
(ii) reacting the product of stage (i) with a base;
(iii) removing aldehydes from the product of stage (ii) and adding an organic phase, followed by distillation or phase-separation to obtain the desired phosphine.
Preferably, the THP has its hydroxy group on the carbon atom which is joined to the phosphorus atom.
Preferably, the removal of aldehydes from the product of stage (ii) is achieved by the addition of sodium sulphite.
The present invention also provides a primary, secondary or tertiary phosphine made by the method described in the immediately-preceding paragraph. Such phosphines include, for example, 1,2-diphosphino-ethane, diethylphosphine and tri-n-butyl phosphine.
Preferably, in stage (i), there is present a stoichiometric excess of the organic halogen-containing compound, relative to the THP. For example, the halogen-containing compound may be present in an amount of up to 10:1, for example 2:1 to 3:1, relative to the THP.
Stage (i) is preferably carried out at a temperature of less than 90xc2x0, at ambient pressure. Stage (i) may be carried out in an inert (e.g. nitrogen or argon) atmosphere. Stage (i) is preferably carried out in the presence of a solvent. The solvent may be water or a water/alcohol mixture, sufficient to solubilise the organic halogen-containing compound and to achieve a practical reaction-rate.
The organic halogen-containing compound may suitably have the general formula R(X)n, where R represents an alkyl, alkenyl, alkaryl, alkynyl or styryl group of from 1 to 20 (preferably 2 to 8) carbon atoms, X represents a halogen (e.g. chlorine, bromine or iodinexe2x80x94preferably bromine) atom and n is a whole number of from 1 to 4, the group R having at least one aliphatic carbon atom adjacent the or each halogen atom.
For example, the organic halogen-containing compound may be ethyl bromide, butyl bromide, 1,2-dibromo-ethane or 1,3-dibromopropane.
The group R may further include one or more ether-or amino-linkages. The base used in stage (ii) may be, for example, sodium hydroxide or potassium hydroxide.
The aldehyde-removing agent used in stage (iii) may be, for example, sodium sulphite. During stage (iii), the pH of the reaction mixture is preferably maintained at 6.0 to 8.0 by the addition of an acid such as hydrochloric acid or phosphoric acid or a suitable organic acid such as acetic acid.
The organic phase added to the reaction mixture during stage (iii) may be a mineral oil (such as paraffin oil) or a petroleum ether of suitable boiling-point range (for example 100-120xc2x0 C.).
While the reaction which is the subject of the present invention has been described herein with particular reference to the use a tris(hydroxyogano) phosphine (THP) as the starting material, it is also possible to obtain the THP in situ by reacting a tetrakis(hydroxyalkyl)phosphonium salt with a base. For example, tris(hydroxymethyl)phosphine can be produced in situ by reacting tetrakis(hydroxymethyl)phosphonium chloride (THPC) or tetrakis(hydroxymethyl)phosphonium sulphate (THPS) with sodium hydroxide.